Coatings are often applied to implantable medical devices to increase their effectiveness or safety. These coatings may provide a number of benefits including reducing the trauma suffered during the insertion procedure, facilitating the acceptance of the medical device into the target site, or improving the effectiveness of the device.
A coating that serves as a therapeutic agent is one such way in which the coating on a medical device can improve its effectiveness. This type of coating on the medical device allows for localized delivery of therapeutic agents at the site of implantation and avoids the problems of systemic drug administration, such as producing unwanted effects on parts of the body which are not being treated, or not being able to deliver a high enough concentration of therapeutic agent to the afflicted part of the body.
Expandable stents are one specific example of medical devices that can be coated. Expandable stents are tubular structures formed in a mesh-like pattern designed to support the inner walls of a lumen, such as a blood vessel. These stents are typically positioned within a lumen and then expanded to provide internal support for the lumen. Because the stent comes into direct contact with the inner walls of the lumen, stents have been coated with various compounds and therapeutics to enhance their effectiveness. The coating on these stents may contain a drug or biologically active material which is released in a controlled fashion (including long-term or sustained release) and delivered locally to the surrounding blood vessel.
Aside from facilitating localized drug delivery, the coating on a medical device can provide other beneficial surface properties. For example, medical devices are often coated with radiopaque materials to allow for fluoroscopic visualization during placement in the body. It is also useful to coat certain devices to enhance biocompatibility or to improve surface properties such as lubricity.
One way in which a coating can be applied to a medical device is to spray the coating substance onto the device using a spray nozzle that atomizes the coating substance. Conventional spray nozzles used in coating medical devices create a wide spray plume. A wide spray plume can result in low transfer efficiencies because only a small amount of the sprayed coating material may be deposited on the medical device. For a small-sized medical device, such as a coronary stent, the transfer efficiency can be very low. Much of the coating solution is lost in excessive overspraying and is therefore wasted. Transfer efficiencies are important as some coating materials are expensive, such as therapeutic agents, drugs and polymers. In addition, the quality of the spray plume from conventional spray nozzles can be inconsistent, causing variability in the thickness of the coating. Thus, the coating may be thicker at one end of the device, or the coating thickness may vary on an individual target-to-target basis, reducing manufacturing reproducibility. Such variability could be detrimental to obtaining uniform coating distribution and thickness on the target, making it difficult to predict the dosage of therapeutic that will be delivered when the medical device or stent is implanted.
Therefore, there is a need for a cost-effective method for improving the performance of spray coating medical devices by reducing the size of the spray plume, which would improve coating transfer efficiency, increase coating uniformity and permit precise control of coating deposition.